optimize set(dict(...)) using set_insert_clean()

Objects/setobject.c

@@ -923,6 +923,8 @@ set_update_dict_lock_held(PySetObject *so, PyObject *other)
     * that there will be no (or few) overlapping keys.
     */
     Py_ssize_t dictsize = PyDict_GET_SIZE(other);
+    if (dictsize == 0)
+        return 0;
     if ((so->fill + dictsize)*5 >= so->mask*3) {
         if (set_table_resize(so, (so->used + dictsize)*2) != 0) {
             return -1;
@@ -933,6 +935,19 @@ set_update_dict_lock_held(PySetObject *so, PyObject *other)
     PyObject *key;
     PyObject *value;
     Py_hash_t hash;
+
+    /* If our table is empty, we can use set_insert_clean() */
+    if (so->fill == 0) {
+        setentry *newtable = so->table;
+        size_t newmask = (size_t)so->mask;
+        so->fill = dictsize;
+        FT_ATOMIC_STORE_SSIZE_RELAXED(so->used, dictsize);
+        while (_PyDict_Next(other, &pos, &key, &value, &hash)) {
+            set_insert_clean(newtable, newmask, Py_NewRef(key), hash);
+        }
+        return 0;
+    }
+
     while (_PyDict_Next(other, &pos, &key, &value, &hash)) {
         if (set_add_entry(so, key, hash)) {
             return -1;